Photodetectors (PDs) are physical devices that are used to measure light's presence by converting energy contained in a light quantum (i.e. a photon) into an electrical form that is easier to measure, amplify, and quantify. The energy conversion process may be indirect in which many photons over a period of time generate a detectable heat amount. The heat is then converted to an electrical signal by a thermocouple junction or a resistive bolometer or other thermodynamic processes. The energy conversion process may, in contrast, be direct in that one light quantum or photon causes temporary matter ionization by removing an electron from a chemically bound state to a free vacuum state. The damaged bond may or may not migrate among neighboring atoms, but may eventually be restored to a full bonding state, although there are exceptions such as f-centers in halide salts.
If the PD is a vacuum photomultiplier tube and an incident photon has sufficient energy, an electron may be ejected from the anode surface and may be collected by an accelerating potential in vacuum. The accelerated electron may collide with other anode surfaces to create an electron avalanche which may be registered electrically as a quantity of charge. The net directional movement of many such charge quanta gives rise to an electric current that can be interpreted as information.
If the PD is a semiconductor device, the ionization process is said to be internal and both the ionized electron and the damaged bond are mobile and can move by scattering from one atom to another in a well ordered crystal lattice. Because the electron and damaged bonding state necessarily have opposite electrical charge, they drift in opposite directions in an electric field. Each contributes to the electrical current generated by an incident stream of ionizing photons. The damaged bond carries a positive charge and is referred to as a “hole”. The electron carries a negative charge. A “hole” current as well as an electron current can be characterized.
PDs available today for the rapid transport of information are the semiconductor type. Semiconductor PDs work on the quantum energy conversion principle described above, with variations designed to improve amplification, light detection efficiency, and fast response to a burst of photons. For example, an inhomogeneous semiconductor junction suffices to effectively separate and collect photo-generated electron-hole current, but is not optimized. An avalanche photodiode (APD) offers greater sensitivity because, the initial ionization charge created by a sufficiently energetic incident photon is amplified by using an electric field acceleration and charge amplification process within the semiconductor that is similar to that occurring in a vacuum photomultiplier tube discussed above. Resonance cavity enhanced (RCE) PDs utilize an enhanced back-side reflection structure to record as much light as possible. A metal-semiconductor junction is an inhomogeneous semiconductor junction that is generally referred to as a “Schottky” junction and is also effective in collecting photo-ionization current that is generated in the semiconductor substrate. The metal-semiconductor-metal (MSM) PD works on the principle of the Schottky junction and is designed primarily for speed.
The P-I-N structure is the basic semiconductor junction structure prevalent today in optical communication and is a good compromise between high speed and good detection efficiency. PDs that have the P-I-N structure are called P-I-N PDs. In P-I-N PDs, the semiconductor ionization and electron-hole generation process occurs in a first chemically pure or intrinsic-type semiconductor layer. A second semiconductor layer is purposely contaminated with atoms that come to equilibrium in the same or similar semiconductor crystal lattice by releasing a spare, mobile electron that is shared by all the atoms. This spare atom is said to occupy states in the conduction band. This second semiconductor layer is called the “n-type” layer. A third semiconductor layer is purposely contaminated with atoms that come to equilibrium in the same or similar semiconductor crystal lattice by trapping electrons from lattice atoms in order to form stable bond. The resulting unpaired bond has a positive charge that is shared by all the atoms and is said to occupy states in the valence band. The third semiconductor layer is called the p-type layer.
Due to the periodic nature of the crystal potential, crystalline semiconductors, metals, and insulators are characterized by bands of states that are distinguished only by small increments of energy and momentum. In addition, crystalline potentials promote the appearance of regions of energy in which stable states are forbidden. This energy distribution is in sharp contrast to the discrete nature of energy states in isolated atoms. Bonding states occupy the valence band and un-bound electrons occupy the conduction band. Separating the two bands is a band-gap in which no stable states exist. In metals, electronic states overlap energetically with the conduction band and the metal is conductive. In semiconductors, when electronic states overlap with the conduction band, the semiconductor is n-type. When hole states, or shared unpaired bonds, overlap energetically with the valence band, the semiconductor is p-type. When the semiconductor is intrinsic or uncontaminated, there are few mobile electrons and holes, only those that are thermodynamically generated by the sample temperature, and the semiconductor is a poor conductor. Crystalline insulators can be characterized as having an energetically high conduction band.
Waveguide-type P-I-N designs are represented in the publication by Vincent Magnin, et al., in the Journal of Lightwave Technology, Vol. 20, p. 477 (2002). These have rapid response when the intrinsic-type region is very narrow, typically ½ to 1 μm in thickness. Unfortunately, this means that the semiconductor waveguide used to channel light into the detection i-type region is the same thickness and unable to collect sufficient light from an optical fiber or polymer waveguide whose core dimensions are typically 9 μm to 50 μm. In this situation, most of the light is lost. This design type has little sensitivity at high speed. If a much thicker semiconductor waveguide is used to channel more light into the detection i-type region, the waveguide-type P-I-N design will have increased sensitivity, but will have a much slower temporal response. This is because the electrons and holes that are photo generated in the intrinsic or undoped-type layer, having a finite field drift velocity, will take a longer time to travel to the p-type and n-type sides and be recorded as a current.
Refraction-type P-I-N designs are represented in the publication by Hideki Fukano et al., in the Journal of Lightwave Technology, Vol. 15, p. 894 (1997). These designs rely on using an oblique light entry facet and Snell's law to guide light to a P-I-N photo-ionization and charge collection region that is located on a different surface. The oblique surface makes a substantial angle with the travel direction for the incoming light and with the planar surfaces containing the P-I-N layer structure and electrical contact pads. Generally, the incident light emanates from a semiconductor laser, optical fiber, or optical waveguide and is usually diverging. Consequently, the oblique surface has to be in close proximity to the area containing the P-I-N layers, referred to as the active area. If the active area is small, as it must be in order to minimize capacitance and promote rapid temporal response, then much of the light incident on the refraction surface will miss the active area and will not be registered, reducing the sensitivity of the refraction-type. If the active area is enlarged in order to collect more of the refracted light, then the speed of the PD is lowered.